1. Field of the Invention
An aspect of the present invention relates to a semiconductor device including a semiconductor region having Ge as a primary component and an insulator film, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
One of technical issues of Ge MOS (Metal/Oxide/Semiconductor) devices is thermal instability in an interface between an insulator and a Ge substrate. Particularly degassing of reducing gas GeO(g) from the interface becomes a problem. Due to the degassing, the characteristics of the interface may deteriorate, and further the mobility may fall down.
One solution has been reported, in which N or Zr is mixed into interface Ge oxide to increase the thermal stability of the interface so that degassing can be suppressed effectively (Kamata, Y., Materials Today (2008) 11, 30). In this method, however, the interface characteristics deteriorate due to the Ge oxide, and the mobility is also low.
As another solution, there has been researched a method in which no thermally instable Ge oxide is formed in the insulator/Ge interface. For example, there has been researched a method for suppressing formation of an interface Ge oxide layer by a method in which Ge3N4 is formed by plasma nitriding (Takagi, S., et al., Microelectron. Eng. (2007) 84, 2314) or a method in which an Si cap layer is formed on a Ge substrate (Kamata, Y., Materials Today (2008) 11, 30). The method of plasma nitriding can form a fresh reaction interface so that it can be expected to improve the interface characteristics. However, there is a fear that the mobility may deteriorate due to nitrogen introduced into the interface. The method of forming Si cap layer can make use of knowledge of insulator/Si gate stacks. However, there is a fear that the mobility may deteriorate due to the existence of the Si cap layer itself, Ge may diffuse into an insulator/Si interface and consequently a Ge oxide interface layer may be formed, the characteristics of an insulator film/substrate interface may deteriorate, or the characteristics may deteriorate due to a defect of an Si/Ge interface.
There is another report about a method in which SrSi2 is put in an insulator film/Si interface in an Si MOS device so as to improve the interface characteristics (Akira Takashima, et al., 13th Workshop on Gate Stack Technology and Physics (2008)). This SrSi2 is not a bulk material but an adsorption layer where SrSi2 is adsorbed in the surface of a semiconductor substrate. As reported in Evers, J. and Weiss, A., Mater. Res. Bull. (1974) 9, 549 (Table 1), SrSi2 is a metal with a band cap of 0, similar to general metal germanides. Akira Takashima, et al., 13th Workshop on Gate Stack Technology and Physics (2008) also says that Sr deposited on a washed Si (2×2) surface shows no reaction with Si even if the temperature of Sr increases, but Sr is desorbed at about 800° C. and only a one-atom layer corresponding to adsorbed Sr stays on the Si substrate.